Impedance pneumography is a known technique of ventilation monitoring, involving the measurement of respiration from electrical impedance changes in the chest. Many investigations have confirmed the high correlation between impedance change and volume of air breathed, whilst the simplicity of this method of respiratory monitoring makes it a very attractive technique as direct contact with the airstream is not required. In practice, the technique is carried out by placing two or four ECG electrodes on the chest wall, and both the electrocardiogram and impedance pneumogram may be recorded from the same electrode set.
Impedance changes, particularly in the chest, are a complex function of geometry and conductivity and many studies have been undertaken in an attempt to understand the relationship between impedance changes and specific physiological changes. In conventional impedance pneumography, little current passes through the lung tissue itself and it is the current passing through the subject's chest and back that contributes most to the impedance change. This is one of the reasons why movement artefact in the resulting pneumogram may be very significant, and in measurements where it is difficult to restrict movement, conventional pnuemographic monitoring can be highly unreliable. In a particular and important application, sleep apnoea detection systems using impedance pneumography techniques give very unsatisfactory results, due to the sensitivity to body movement. Changes in body shape and/or in the skin-to-electrode impedance can produce large amplitude artefacts, in many cases larger than the respiratory signal itself. In neonates and infants especially, the impedance fluctuations caused by cardiac activity can be misclassified as breathing, which might falsely prevent an alarm if obstructive apnea occurs.
A recent investigation into transthoracic impedance, "Signal-to-Motion Artefact Ratio Versus Frequency for Impedance Pneumography", Rosell & Webster, IEEE Transactions on Biomedical Engineering, Vol. 42, No. 3, March 1995, pages 321-323, demonstrated the frequency-dependent aspects of pneumograph measurements. The study described concluded that working at higher frequencies improves the signal-to-motion artefact ratio and suggested a method to further increase this ratio. Development of this study has led to the design and testing of an adaptive filter to increase the signal-to-motion artefact ratio ("Reduction of Motion Artefact Using a Two-Frequency Impedance Plethysmograph and Adaptive Filtering", Rosell, Cohen & Webster, IEEE Transactions on Biomedical Engineering, Vol. 42, No. 10, October 1995, pages 1044-1048).
Other studies into multifrequency impedance monitoring have led to the finding that the amplitude of the transthoracic impedance (measured using a four-electrode system) decreases with frequency ("Multifrequency data collection and modelling of cardiac and respiratory related electrical impedance changes", Brown, Lu, Smallwood & Leathard, in Concerted Action on Electrical Impedance Tomography, Barcelona meeting, UPC, Barcelona 1993).